tests: make simplecov/coveralls optional

If we do not have SimpleCov or Coveralls installed, we should be able to
continue with the rest of the test suite.

This allows the tests to run outside of Bundler if SimpleCov or
Coveralls are not installed.

spec/spec_helper.rb

@@ -1,3 +1,4 @@
+begin
   require 'simplecov'
   require 'coveralls'
 
@@ -16,6 +17,9 @@ if !RUBY_VERSION.start_with? '1.8'
     Object.send(:remove_const, :Docile)
     $LOADED_FEATURES.reject! { |f| f =~ /\/docile\// }
   end
+rescue LoadError
+  warn 'warning: simplecov/coveralls gems not found; skipping coverage'
+end
 
 lib_dir = File.join(File.dirname(File.dirname(__FILE__)), 'lib')
 $LOAD_PATH.unshift lib_dir unless $LOAD_PATH.include? lib_dir